In 2001, Mayes et al. described a high voltage Marx generator sourcing a helical antenna. “The Marx Generator as an Ultra Wideband Source,” J. R. Mayes, W. J. Carey, W. C. Nunnally, and L. Altgilbers, 13th IEEE International Pulsed Power Conference, 2001. While the helical antenna is traditionally a resonant device, the paper described what is now described as an impulse excitation, or shock-exciting-antenna structure, such that the structure rings with several cycles at a frequency defined by the antenna geometry. The paper described a Marx generator capable of delivering 125 kV, sourcing a helical antenna designed for a resonant frequency of 1 GHz. The paper concluded its discussion of the Marx-helical device with a measured waveform and calculated spectral response. The radiated electric field was witnessed to be less than expected. The electric field strength was measured to be only approximately 300 V/m, measured at 100 m, as shown in FIG. 2(a). However, the spectral response, shown in FIG. 2(b) was as expected, with a center frequency of 1 GHz.
Shock excitation of a resonant structure results in a brief voltage ring, producing several cycles of energy that are radiated by the structure. This is an inefficient method for producing wideband energy because the driving pulsed power produces an Ultra Wide Band spectrum of energy, as shown in FIG. 3(a). The helical antenna can be viewed as a bandpass filter in which the structure radiates, or passes energy, at a center frequency, as well as energy located in frequencies near and around the center frequency; thus acting as a wide band filter as illustrated in FIG. 3(b). Energy not radiated by the antenna is either recaptured by the source or, in the case of a Marx generator-fed system, the energy is dissipated via heat.
A better description of the shock-excited helical antenna, however, is one in which the physical parameters, including the stray capacitance and inductance, are pulse-excited with a voltage. The relaxation of these parameters occurs at the antenna's natural resonant frequency. Thus the radiation from the helical antenna manifests as an impulse response, and, with extremely high impulse voltages, the radiated electric field can be high.
Analysis of the initial effort showed that the primary cause of the inefficiency in the peak electric field strength was the transition section between the Marx generator and the helical antenna, as well as the initial geometry of the helical antenna. In the initial demonstration a coaxial section interconnected the Marx generator to the input of the helical antenna, as suggested by J. D. Kraus. John D. Kraus, Antennae, 2nd edition, McGraw-Hill Inc., 1988. As shown in FIG. 4, the helical antenna conductor immediately begins its spiral path from the coaxial geometry of the feed structure. This abrupt transition from a coaxial geometry to the coil geometry results in a high inductance that produces an excessive impedance mismatch, as well as an enhancement point that results in a corona emission with high voltages and ultimately results in the failure of the antenna.